In wireless mobile communications, a channel that couples a transmitter to a receiver is often time-varying due to relative transmitter-receiver motion and multipath propagation. Such a time-variation is commonly referred to as fading and may severely impair system performance. When a data rate for the system is high in relation to channel bandwidth, multipath propagation may become frequency-selective and cause intersymbol interference (ISI). By implementing inverse fast Fourier transform (IFFT) and cyclic prefix (CP) insertion at the transmitter, together with CP removal and FFT processing at the receiver, orthogonal frequency-division multiplexing (OFDM) converts frequency-selective ISI channels into a set of parallel flat fading subchannels. Each subchannel can subsequently be equalized with significantly reduced complexity. As a result, OFDM has found widespread applications in digital subscriber lines (DSL), digital audio/video broadcasting (DAB/DVB), and wireless local area networking (LAN) standards, including IEEE802.11a and Hiperlan/2.
Being OFDM's counterpart for multi-user communications, orthogonal frequency-division multiple access (OFDMA) inherits OFDM's ability to effectively mitigate ISI that arises when high-rate transmission propagate through time dispersive or frequency selective channels. Originally proposed for cable TV networks, OFDMA is now being considered for IEEE 802.16a, ETSI Broadband Radio Access Networks (BRAN), and multi-user satellite communications.
In its simplest form, each OFDMA user transmits information symbols using one complex exponential or subcarrier that retains orthogonality with other users' subcarriers when passing through multipath fading channels. As a result, multi-user interference (MUI) can be suppressed deterministically, regardless of the underlying ISI channels. In uplink applications, this one-carrier-per-user access scheme is also power efficient because each user maintains a constant modulus transmission. However, the performance of OFDMA can be significantly reduced when a user-specific channel exhibits deep fades or nulls at the information-bearing subcarrier. Error-control coding and/or frequency hopping are usually employed to rubustify performance against channel fades. Alternatively, multiple subcarriers can be assigned per user to support high data rate applications at the expense of non-constant modulus signaling. However, the multicarrier OFDMA scheme reduces efficiency of the power amplification stage at the transmitter.